HCV is an increasing cause of morbidity and mortality in China, with an estimated per capita prevalence that is three-fold that of the United States. Although there are limited data, HCV incidence is also thought to be increasing. However, the contribution of unsafe sex to ongoing HCV transmission among high-risk groups such as MSM (men who have sex with men) is not well understood. Meanwhile, China's estimated three to 18 million MSM exhibit very high HIV incidence and prevalence rates. The number of HIV-positive Chinese MSM is expected to increase due to high rates of unprotected anal intercourse and the high incidence of HIV infection. Most studies on HCV in China examine parenteral causes of transmission such as intravenous drug use (IDU), plasma blood donation, or medical treatment-related infections, and have not focused on sexual transmission of HCV. Recent studies in Western countries have shown an epidemic of HCV infection among MSM that is unrelated to parenteral spread. Studies suggest sexual transmission of HCV, especially among HIV-infected MSM, due to rectal co-infections and/or rough sex leading to mucosal breakdown. There are also respondent driven sampling data from MSM in China suggesting a high rate of intimate partner violence, which may also contribute to homosexual HCV transmission. By exploring the HCV status of partners of established HIV-infected MSM, we aim to study the contribution that sexual activity plays in HCV transmission. Therefore, using a mixed-method design with a known clinical population (i.e., HIV-positive Chinese MSM), we propose to (1) establish the differential and joint contribution of sexual risks (e.g., unprotected, receptive anal intercourse) and IDU to HCV transmission among HIV-infected Chinese MSM and their IDU and sexual partners; (2) establish the molecular epidemiology of HCV and HIV (via genotyping and phylogenetic analysis); describe and examine biosocial correlates (e.g., viral load; multiple sexual partners; sexual risks; multiple drug-using partners, cross-generational drug use) associated with lack of transmission via these two routes; (4) describe and examine behavioral and social network characteristics (egocentric) associated with sexual and/or drug risk determinants of HCV; and (5) model expected HCV-HIV morbidity based on these findings. Our study has high public health significance because, with the recent advent of new therapies for chronic HCV treatment (such as direct-acting agents), HCV has become a highly treatable disease yet many infected persons remain unaware of their condition. The study is innovative in that it delves more deeply into specific routes of sexual transmission, whereas many previous studies classified all sexual partners without distinction. The integration of behavioral network analysis and HCV and HIV phylogenetic analysis provides enhanced evidence for developing tailored prevention strategies for HCV-HIV co-infected MSM.